The present invention concerns a magnesium treatment process for the production of a treated molten mass with less than 100 grams per ton of non-metallic inclusions, and an apparatus for carrying out this process.
It is known to treat a molten iron mass with magnesium with the object of influencing the graphite morphology in such a way that spheroidal graphite is formed. In such a process fairly large quantities of non-metallic compounds occur, for example oxides, sulphides and similar. This effect is a result of the high affinity of the magnesium for oxygen and sulphur.
The non-metallic reaction products are mostly eliminated during the treatment of the molten iron mass, or in the time interval between the end of treatment and decantation, as a result of the differences between the density of the slag particles (approximately 3,000 kg/m.sup.3) and the molten mass (approximately 7,000 kg/m.sup.3). However, a portion of the reaction products, particularly particles smaller than 12.5 .mu.m, still remains in suspension in the molten mass, even after the usual time available.
The flotation speed can be determined with relatively good precision by Stokes' equation. ##EQU1## where: L=Flotation height in mm
d=Particle size in um PA1 .delta.Fe=Density of iron in Kg/m.sup.3 (7,000) PA1 .delta.slag=Density of slag in Kg/m.sup.3 (3,000) PA1 t=Flotation time in seconds PA1 .mu.=Dynamic viscosity (0.007)
Practical measurements showed that, with conventional treatment processes, the quantity of particles in suspension is between 200 and 600 grams per ton of treated iron, and this quantity is then poured with the pouring stream into the casting mold. The tests also showed that these inclusions, which remain in the casting, can substantially reduce the dynamic properties of the casting.
In order to enable the flotation of the reaction products the molten mass would have to be left to stand in the pan for a lengthy period. However, this would lead to considerable, generally unacceptable temperature losses (6.degree.-15.degree. C. per minute).
The use of the process, known from steel production, of rinsing the molten mass with inert gas (N, Ar, etc.), causes an increased oxidation of the magnesium, and this leads to the formation of further, new inclusions, which means that the desired object cannot be achieved with this process.